There has been widely known a method in which a single layer made of a polyvinyl alcohol type resin (hereinafter referred to as “PVA type resin”) and formed in a film shape is subjected to dyeing and stretching to produce a polarizing film comprising a PVA type resin layer, wherein molecules of the PVA type resin are oriented in a direction of the stretching, and a dichroic material is impregnated in the PVA type resin in a molecularly oriented state. The polarizing film to be obtained by the above conventional method using a PVA type resin single layer film usually has a thickness in the range of about 15 to 35 μm. The conventional method makes it possible to obtain a polarizing film having the optical characteristics of a single layer transmittance of 42% or more; and a polarization rate of 99.95% or more. Currently, polarizing films produced by the conventional method are used in optical display devices for televisions, and other appliances.
It should however be noted that a PVA type resin is hydrophilic and highly hygroscopic, so that a polarizing film produced using the PVA type resin is sensitive to changes in temperature and humidity, and more likely to expand and contract due to changes in surrounding environments and is liable to be subjected to occurrence of cracks. Moreover, the expansion and contraction caused by environmental changes during use will produce stress applied to an adjacent member to which the polarizer film is joined, and thereby cause deformation, such as warp, in the adjacent member.
Thus, in order to suppress such expansion and contraction of a polarizer film to thereby reduce the influence of changes in temperature and humidity, it has been a usual practice to employ, in the case of a polarizing film for use in televisions, a laminate prepared by laminating a triacetylcellulose (TAC) film having a thickness of 40 to 80 μm and serving as a protection film, on each of opposite surfaces of a polarizing film. However, even in such a structure, in cases where a single layer polarizing film is used therein, because of a limit in reducing the thickness of the polarizing film, expansion and contraction forces produced in the polarizing film are of a level which cannot be ignored, so that it is difficult to completely avoid the influence of expansion and contraction of the polarizing film, and a certain level of expansion and contraction will inevitably occur in the optical film laminate including the polarizing film. If expansion or contraction occurs in such an optical film laminate including a polarizing film, stress arising from the expansion or contraction will cause deformation, such as warp, in an adjacent member. This deformation, even if it is small, leads to the occurrence of non-uniformity of display in a liquid-crystal display device. Thus, in order to suppress the occurrence of non-uniformity of display, it is necessary to make design considerations, for example, to carefully select a material for each member to be used in the optical film laminate including the polarizing film. Further, the stress produced by the contraction of the polarizing film will cause the optical film laminate being peeled off the liquid-crystal display panel, so that there will be a need to use an adhesive having a high adhesive power to join the optical film laminate to the liquid-crystal display panel. However, the use of such a high-power adhesive gives rise to a problem of difficulty in re-working which is an operation of, when the presence of an optical defect is found in a polarizing film of an optical film laminate laminated to a liquid-crystal display panel through a subsequent inspection, peeling the optical film laminate from the liquid-crystal display panel and laminating another optical film laminate to the liquid-crystal display panel. This is a technical problem encountered in a polarizing film produced by the conventional method using a single layer of a PVA type resin formed in a film shape.
Thus, there is a need for a new method of producing a polarizing film, as an alternative to the conventional polarizing film production method using a PVA type resin single layer, which is incapable of reducing the thickness of a polarizing film to a sufficient level due to the aforementioned problem. Specifically, it is practically impossible to produce a polarizing film having a thickness of 10 μm or less by the conventional method using a PVA type resin single layer formed in a film shape. This is because, in producing a polarizing film using a single layer of a film-shaped PVA type resin, if the thickness of the PVA type resin single layer is excessively reduced, dissolution and/or breaking is likely to occur in the PVA type resin layer in the dyeing step and/or the stretching step, so that it becomes impossible to form a polarizing film having a uniform thickness.
In order to cope with this problem, there has been proposed a method designed such that a PVA type resin layer is applied and formed on a thermoplastic resin substrate, and the PVA type resin layer formed on the resin substrate is stretched together with the resin substrate, and subjected to dyeing, so as to produce a polarizing film significantly thinner than the polarizing film obtained by the conventional method. This polarizing film production method using a thermoplastic resin substrate is noteworthy in that it provides a possibility of producing a polarizing film more uniformly than the polarizing film production method using a PVA type resin single layer.
For example, Japanese Patent JP 4279944B (Patent Document 1) discloses a polarizing plate production method which comprises steps of forming a polyvinyl alcohol resin layer having a thickness of 6 μm to 30 μm, on one of opposite surfaces of a thermoplastic resin film by a coating process, stretching the polyvinyl alcohol resin layer at a stretching ratio of 2.0 to 5.0 in such a manner that the polyvinyl alcohol resin layer is formed as a transparent coating element layer to thereby form a composite film consisting of two layers, the thermoplastic resin film and the transparent coating element layer; laminating an optical transparent resin film layer on the side of the transparent coating element layer of the composite film consisting of the two layers, through a bonding agent, peeling and removing the thermoplastic resin film, and dyeing and fixing the transparent coating element layer in such a manner that the transparent coating element layer is formed as a polarizing element layer. A polarizing plate to be obtained by this method has a two-layer structure consisting of the optical transparent resin film layer and the polarizing element layer. According to the description of the Patent Document 1, the polarizing element has a thickness of 2 to 4 μm.
The method disclosed in the Patent Document 1 is designed to perform stretching under an elevating temperature by a uniaxial stretching process, wherein the stretching ratio is restricted to the range of 2.0 to 5.0, as mentioned above. As for the reason why the stretching ratio is restricted to 5.0 or less, the Patent Document 1 explains that stretching at a high stretching ratio of greater than 5.0 makes it extremely difficult to maintain stable production. Specifically, there is described that the ambient temperature during stretching is set to 55° C. in cases where ethylene-vinyl acetate copolymer is used as the thermoplastic resin film, to 60° C. in cases where non-stretched polypropylene is used as the thermoplastic resin film, or to 70° C. in cases where non-stretched nylon is used as the thermoplastic resin film. The method disclosed in the Patent Document 1 employs a uniaxial stretching process in air under an elevated temperature. Further, as described in the Patent Document 1, the stretching ratio is restricted to 5.0 or less. Thus, a polarizing film having an extremely small thickness of 2 to 4 μm, to be obtained by this method, is not enough to satisfy optical characteristics desired for a polarizing film to be used, for example, in an optical display device such as a liquid-crystal television.
The method of forming a polarizing film with steps of forming a PVA type resin layer on a thermoplastic resin substrate by a coating process, and stretching the PVA type resin layer together with the substrate is also disclosed in Japanese Patent Laid-Open Publication JP 2001-343521A (Patent Document 2) and Japanese Patent Laid-Open Publication JP 2003-043257A (Patent Document 3). The methods disclosed in the Patent Documents 2 and 3 are designed such that a laminate consisting of a thermoplastic resin substrate and a PVA type resin layer applied on the substrate is subjected to uniaxial stretching at a temperature of 70° C. to 120° C., in cases where the substrate is made of a non-crystallizable polyester resin. Then, the PVA type resin layer molecularly oriented by the stretching is subjected to dyeing to allow a dichroic material to be impregnated therein. In the Patent Document 2, there is described that the uniaxial stretching may be longitudinal uniaxial stretching or may be transverse uniaxial stretching. Differently, in the Patent Document 3, a method is described in which transverse uniaxial stretching is performed, and, during or after the transverse uniaxial stretching, contracting the length in a direction perpendicular to a direction of the stretching by a specific amount. In both of the Patent Documents 2 and 3, there is described that the stretching ratio is typically set to about 4.0 to 8.0. Further, there is described that the thickness of a polarizing film to be obtained is in the range of 1 to 1.6 μm.
In the Patent Documents 2 and 3, although there is described that the stretching ratio is typically set to 4.0 to 8.0, since the Patent Documents 2 and 3 adopts an elevated temperature in-air stretching process, it is considered that the stretching ratio will be limited to 5 as described, for example, in the Patent Document 1. None of the Patent Documents 2 and 3 describes a specific technique for achieving a stretching ratio of greater than 5.0 by the elevated temperature in-air stretching process. In fact, in Examples described in the Patent Documents 2 and 3, only a stretching ratio of 5.0 and a stretching ratio of 4.5 are described, respectively. Through additional tests on the methods disclosed in the Patent Documents 2 and 3, the inventors of the present invention have ascertained that it is impossible to adequately perform stretching at a stretching ratio of greater than 5.0 by the methods disclosed therein. Therefore, it should be understood that only a stretching ratio of 5.0 or less is substantially disclosed in the Patent Documents 2 and 3. As with the Patent Document 1, the polarizing film to be obtained by the method disclosed in each of the Patent Documents 2 and 3 is not enough to satisfy optical characteristics desired for a polarizing film to be used, for example, in an optical display device such as a liquid-crystal television, etc.
U.S. Pat. No. 4,659,523 (Patent Document 4) discloses a polarizing film production method which comprises subjecting a PVA type resin layer coated on a polyester film to uniaxial stretching together with the polyester film. This method disclosed in the Patent Document 4 is intended to form the polyester film serving as a substrate of the PVA type resin layer in such a manner as to have optical characteristics allowing the polyester film to be used together with a polarizing film, but it is not intended to produce a polarizing film comprising a PVA type resin layer and having a small thickness and excellent optical characteristic. Specifically, the method disclosed in the Patent Document 4 is no more than a technique of improving optical characteristics of a polyester resin film to be stretched together with a PVA type resin layer to be formed as a polarizing film. A polarizer material production method having the same object is also disclosed in Japanese Patent Publication JP 08-012296B (Patent Document 5).